1. Field of the Invention
The present invention relates to an amplifier circuit with a feedback path.
2. Description of the Related Art
Amplifiers are increasingly used in environments with high temperature fluctuations. For example, the specified temperature ranges for amplifier circuits employed in the automobile area are between −40° C. and 125° C. Here, amplifier circuits exhibiting constant amplification behavior over a great temperature range are required.
Frequently, so-called low-noise amplifiers or LNAs are employed as amplifiers. These LNAs have reduced noise behavior, which is why they are able to amplify signals, without significantly increasing the noise portion of the signals.
FIG. 4 shows a possible amplifier circuit with a low-noise amplifier. In this circuit, a constant current source 1 is attached to a collector or a first terminal of a current mirror transistor 3. At the same time, the constant current source 1 is connected to a control terminal or a base of the current mirror transistor 3 via a current mirror resistor 5. The base of the current mirror transistor 3 is attached to a base or a control terminal of an input transistor 9, which forms the current mirror together with the transistor 5, via the current mirror resistor 5 and an input resistor 7. The constant current source 1 is connected to an emitter or a second terminal of the current mirror transistor 3 as well as to ground via a shunt impedance 11. An emitter or a second terminal of the input transistor 9 is conductively connected to the emitter of the current mirror transistor 3 and attached to ground.
An input terminal 13 is connected to the base of the input transistor 9 and coupled to the base of the current mirror transistor 3 via the resistors 5, 7. A collector or a first terminal of the input transistor 9 is coupled to an emitter or a second terminal of an output transistor 15. A collector or a first terminal of the output transistor 15 is coupled to an output terminal 19 via an output capacitor 17.
The output terminal 19 is coupled to a base or a control terminal of the output transistor 15 via a first feedback capacitor 21, a feedback resistor 23, and a second feedback capacitor 25. At the same time, the base of the output transistor 15 is connected to ground via the second feedback capacitor 25 and a feedback inductance or a feedback inductive element 27.
A bias terminal 29 is conductively connected to the base of the output transistor 15. The collector of the output transistor 15 is connected to a supply voltage terminal 35 via a supply voltage resistor 31 and a supply voltage inductance 33.
The current mirror transistor 3 and the input transistor 9 are arranged in a so-called common emitter arrangement, so that current fed into the circuit from the constant current source 1 at the current mirror transistor 3 is approximately as high as current at the collector of the input transistor 9. The current mirror resistor 5 and the input resistor 7 are designed in high-ohmic manner, so as to diminish reverse influences of signal fluctuations at the input transistor 9 on the current mirror transistor 3. Via the current from the constant current source 1, the working point of the input transistor 9 may be adjusted.
The input transistor 9 is connected between the supply voltage terminal 35 and ground in a cascode circuit with the output transistor 15.
A supply voltage potential is present at the supply voltage terminal 35, whereas a bias is present at the bias terminal 29, if required, via an impedance not shown here.
An alternating voltage signal is applied at the input terminal 13 and amplified by the cascode circuit consisting of the input transistor 9 and the output transistor 15. The signal amplified by the cascode circuit is tapped at the output terminal 19, wherein the output capacitance 17, among other things, serves to filter out potential DC components from the alternating voltage signal at the output terminal.
Via the current from the constant current source 1, the working point and the gain of the cascode circuit are at first predefined. The cascode circuit, the supply voltage inductance 33, and the supply voltage resistor 31 are connected in series between the supply voltage terminal 35 and ground, wherein the supply voltage inductance 33 serves to filter out the reverse influences on the supply voltage terminal 35, which occur in the amplification of an alternating voltage applied at the input. Moreover, the working point and the gain of the cascode circuit may be adjusted via the potential present at the bias terminal 29.
Moreover, via the first feedback capacitor 21, the feedback resistor 23, and the second feedback capacitor 25, the potential present at the output terminal 19 is fed back to the base of the output transistor 15, so that the gain performance of the output transistor 15 is stabilized so as to obtain, for example, a stability factor greater than 1, e.g. in a frequency range up to 10 GHz. The feedback inductance 27 represents a high impedance for the high-frequency alternating current signal in the feedback network.
Among other things, the supply voltage resistor 31 serves for stabilizing the cascode circuit consisting of the input transistor 9 and the output transistor 15.
The amplifier circuit shown in FIG. 4 amplifies alternating voltage present at the input terminal 13. The amplified alternating voltage signal is present at the output terminal 19. Via the value of the direct current provided from the constant current source 1, the working point and thus the gain of the amplifier circuit are preset. Furthermore, the gain of the amplifier circuit is influenced or regulated by a portion of the alternating voltage signal fed back to the base of the output transistor 15 at the output terminal 19. The gain of the amplifier circuit may be varied by suitable choice of the feedback.
In amplifier circuits or LNA concepts, as they are shown in FIG. 4, a simple common emitter structure is employed. In order to minimize the current consumption in mobile systems, the LNAs are supplied with current there, so as to obtain almost temperature-independent overall current consumption.
It is disadvantageous in the amplifier circuit shown in FIG. 4 that it is supplied with constant current, whereby the gain of the amplifier varies over the temperature. This variation is undesirable or too great in some applications.
In the amplifier circuit shown in FIG. 4, it would of course be possible to correctively regulate the gain at a temperature increase via an increase of the constant current from the constant current source 1. This would, however, be connected with increased current consumption of the amplifier circuit, which is disadvantageous particularly with battery-operated applications.